As QF009 has posted, the problem with your itinerary is that you can't have a second entry into Europe unless you're transiting and going to certain African countries only (& MRU isn't one of them).
Your itin. would be valid if you were to just transit LHR on your way from NRT to, say, NBO or DAR, and didn't include the side trip to DXB. (There is no need to return to MRU - any African destination is OK, although only about half a dozen of them are allowed for the purposes of the rule covering the second entry into Europe.)

If you take a look at MileageMonkey http://slfft.allhyper.com/0.95/award...HR-DXB-LHR-MRU you will see the error. Play around with the tool & you can get some excellent info.

The simple solution is to go to DXB (or my preference is for MCT) on the first trip via 'Europe', and finish off in DAR. This becomes http://slfft.allhyper.com/0.95/award...IN-NRT-LHR-DAR which is valid.